The engine/transmission mounts of compact and light-weight automobiles, and particularly those having four cylinder engines, may be required to fulfill differing and somewhat conflicting design requirements. To minimize transmission to the frame of vibrations produced by the engine at idling or low speed operation, it may be desirable for such a mounts to possess low stiffness at low frequencies (e.g., 0-30 Hz). On the other hand, it is also usually necessary for prevention of engine "bounce" and the like that the mounts exhibit a high degree of "damping", or more accurately a large loss angle, at the relatively low frequencies of excitations produced by the road surface over which the automobile travels. The mount should also prevent insofar as possible the production within the vehicle interior of so-called "drone" noise. Such noise occurs when excessive mount stiffness results in transmission to the automobile frame of high frequency (e.g., greater than 100 Hz) vibrations caused by second or subsequent order firing disturbances of the engine during operation at speeds of more than about 3,000 RPM. The objectionable drone noise can be avoided by causing the mount to have a very low dynamic stiffness at the particular high frequency excitation resulting from the engine firing disturbances.
While previously proposed fluids mounts alleviate to some extent the problem of excessive mount stiffness at the high frequency excitations causing drone noise, none are readily tunable or adjustable so as to permit precise control over and selection of the particular high excitation frequency or frequencies at which minimum dynamic stiffness is achieved.